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 Post subject: Re: National SW-3 Thrill Box
PostPosted: Sep Mon 13, 2021 2:27 pm 
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Joined: Mar Wed 16, 2011 10:44 pm
Posts: 2067
Location: Peekskill, NY
The last set of coils pictured, look to be original.

You can check the dimensions here:


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 Post subject: Re: National SW-3 Thrill Box
PostPosted: Sep Sun 26, 2021 4:43 pm 
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Joined: Oct Wed 01, 2014 1:20 pm
Posts: 543
Location: Wood River, Ill.
It has been a week or so since my last post. Jim, Thank You for the coil form drawing. That will come in handy if I ever get around to building a set of coils. (I need to dig out the Southbend and oil it up good first! :D )

Yesterday, I let my curiosity get the best of me and hooked the SW-3 up to my ARBE-III to see what, if anything, I would get out of the radio. The filament supply of the ARBE-III is rated for 3 amps, so I initially adjusted the "A" supply for 2.5 Volts and connected up for 135 Volts B+. This brought the radio to life. I am using Trimm cans for the output. The 2.5 volts on the A supply was causing the ARBE-III to get pretty warm, so I dialed the A supply down to about 2 volts, which brought the current down to around 2.8-2.9 amps. Supposedly the ARBE-III can run at this current draw.

With these voltages, running on the original tube set that came with the radio, I do get signals in the BC band on a relatively short piece of wire for an antenna. The local 50 kW station, about 20 miles away, can be dialed in almost anywhere on the dial, but after adjusting the regen control, I can tune around and find some of the other local stations on the lower end of the BC band.

I may take it up to the shack and connect to a real antenna, and see what I can do with it. It may just load up with the 50 kW at 1120 kC, blocking all others, but the way it is working down here in the shop, I don't think so.

I do get a pretty dominant 120 cycle hum, though. When I started, it did not. Seems like something changed while listening. I suspect I have a leaking cap somewhere, but haven't checked it out yet.

I may see about obtaining the 6 v tubes, as recommended, and play around with that a little as well. For now, I am satisfied, if not "thrilled" that it does work, and seems to work well.

_________________
73
Chris
N9WHH


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 Post subject: Re: National SW-3 Thrill Box
PostPosted: Sep Mon 27, 2021 12:37 am 
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Joined: May Fri 01, 2015 2:33 am
Posts: 1803
Location: Roslyn,Pa 19001
On the hum you might notice a difference with the lid closed vs open. On mine there is. Good earth ground is a good thing. Your p/s my not like the load from the heaters.
I was working on a Philco 16 trying to solve an osc problem and wanted to try a 76 with a little better characteristics. Had a dud 5pin base, 7 pin socket and a 6C4. 15 min later I had what I wanted. The solution to the osc was a missing resistor from the factory. Anyway if you have the spares it's a quick way to replace the 27 with a 6v tube. If you have 6V tubes in your HRO they have a handful of 6D6'S.

Terry


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 Post subject: Re: National SW-3 Thrill Box
PostPosted: Sep Mon 27, 2021 12:49 am 
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Joined: May Fri 01, 2015 2:33 am
Posts: 1803
Location: Roslyn,Pa 19001
OBTW Try dropping this sell a note https://www.ebay.com/str/atlanticelectr ... =461375419 He was making SW-3 forms and is currently making standard 6pin forms.

Terry


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 Post subject: Re: National SW-3 Thrill Box
PostPosted: Oct Mon 04, 2021 4:03 am 
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Joined: Oct Wed 01, 2014 1:20 pm
Posts: 543
Location: Wood River, Ill.
Thank you again for the suggestions, Terry. :D

This weekend, I rounded up a pair of 6D6 and a 37 and installed them into the SW-3. I swapped out the dial lamp for a 6v bulb and powered up using the ARBE-III. I used a pair of Brandes cans on the output. (Much easier on the sunburned ears than the Trimm cans I was using before.) This time the hum was minimal and the receiver worked just as well, if not a little better, than it did with the original tubes that were in it. As I stated before I only have the #67 coil set, so I am limited to the lower BC band. It will pick up the local 50kW station at 1120 KC when the regen control is set at one end of the rotation. Otherwise, when I get the regen control set I can tune in the lower end of the BC band, for the most part. I canā€™t get the local station at 550 KC, though, for some reason. I think some more playing around with a proper antenna may be called for. I pick up plenty above 550, though.

One thing I noticed is that the main tuning dial stops short of giving me the full tuning dial rotation. The tuning capacitor does not rotate a full 180 deg. There is a mechanical stop at both ends of rotation. The Velvet Vernier will easily slip on the shaft of the tuning cap making it difficult to feel the hard stop of the tuning cap. The shaft turns smoothly, but does have some drag. I have never taken one apart, so do not know what the source of drag might be. I suspect that some lubrication and possibly cleaning is in order. I have not yet pulled it off the receiver yet to take a look. It looks like a pinch wheel drive, from what I have seen. The friction may be in the capacitors too. Will investigate.

I did contact the seller you recommended for the coil forms, Terry. He says the materials he uses are difficult to get right now, but said he would let me know when they become available. I will just wait and see where that goes.

For now I will enjoy the thrill of the lower BC band, and the little receiver I now have. :D

_________________
73
Chris
N9WHH


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 Post subject: Re: National SW-3 Thrill Box
PostPosted: Oct Mon 04, 2021 1:26 pm 
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Joined: Mar Wed 16, 2011 10:44 pm
Posts: 2067
Location: Peekskill, NY
I just built my coil sets from LE (linen-based) phenolic, from mcmaster-carr. Just tubing stock that I glued the bottoms into. No raised rim at the top but just a turned groove to allow an easy grasp. Not the special mica-filled National material but they work fine.


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